Electric power tools for professionals have advantages such that their power supply has longer battery life and more power, but have a disadvantage such that they are expensive. Therefore they often become a good target for stealing and reselling. Their size is small enough to be carried by hand, which makes them easy to steal. Also it is often the case that they are taken away by someone by mistake, or that they are just misplaced somewhere and lost. As a result, even if they are not a target of stealing and reselling at the beginning, they end up becoming a good target for reselling.
Moreover, main users of electric power tools are people from construction industries and upholstery industries. People of these industries are often of small businesses. It is often the case that electric power tools do not belong to their company but belong to them personally. Especially, the working environment for these people from construction industries and upholstery industries is such that many people work together at the same place. They have to make sure by themselves that they don't have their electric power tools stolen or lost.
Electric power tools with batteries now have longer battery life and higher battery power because of adoption of lithium-ion batteries. Such a tool carries a risk of a serious accident triggered by a battery fire incident because of its high energy when a battery (which is recommended for use by the manufacturer of the tool) is defective or when a reproduced battery or a compatible battery, which is different from the recommended battery, is used.
There are several methods to prevent having electric power tools stolen, such as:    (1) a method of fixing an electric power tool to some fixed object, using something like a hard wire;    (2) a method of locking and unlocking by entering a correct sequence of numbers through key operations; and    (3) a method of locking and unlocking by touching some electrical key, such as a wireless remote control, an IC card, or near field RFID card.
But the above method (1) does not work if there is no fixed object around to be fixed to the electric power tool, and it is required to buy something like a hard wire, which is a financially heavy burden for the user. The user has to fix the electric power tool to some fixed object every time he/she leaves it, which is cumbersome for him/her.
The above methods (2) and (3) may have a good effect making those people, who try to steal the electric power tool, give up stealing because they may think, even if they successfully steal the electric power tool, they cannot use it because it is locked.
But the above method (2) has never been commercially realized because of its operational difficulties of entering the key and its additional cost in the mean time, Patent Document 1 discloses such a configuration that, as an example of the above method (2), using an identification code input as a key input, switching between the normal operation mode and the operation prevented mode, a user can use the electrical power tool only when the correct key is entered.
Also, the above method (3) has never been commercially realized because of its additional cost for additional apparatuses such as a remote controller or a card, or because of the reason that it is easy to forget to carry the remote controller or it is easy to lose it.
In the mean time, in order to prevent accidents caused by a low quality battery, it is required to have a function to check whether the battery is recommended by the manufacturer or not. The above Patent Document 1 discloses a configuration of setting the same ID number in both the removable unit (such as a battery) and the tool body. If the two ID numbers of the removable unit and the tool body are not the same, then the mode is switched to the operation prevented mode.
However, the configuration disclosed in Patent Document 1 may not work well with the reproduced battery, in which the battery cell is changed and the battery circuit is not changed. Also, this configuration may not k well with the case where more than one of the batteries are used with one tool body or where one battery is used with many different tool bodies.
The present invention is proposed to solve these problems. It is a general object of the present invention to provide a method to prevent having electric power tools stolen, by using an operationally advantageous electric key. Another object is to make it difficult to forget or lose the electric key. Yet another object is to handle various combinations of the battery pack and the tool body. Yet another object is to effectively prevent accidents caused by low quality batteries, and to use a small number of additional parts for achieving the low cost.